It is generally known that steel sheets are normally enamelled as follows:
a) cutting to size of the generally oiled and, in special cases also dried sheets PA1 b) shaping of the sheets, optionally after application of drawing aids, into workplaces, for example by deep drawing PA1 c) chemical pretreatment of the workpieces, for example by degreasing, pickling, nickel plating and/or passivation PA1 d) application of the enamel frit by known methods, for example by electrostatic powder coating (in which case step c) may be partly omitted) or slip coating PA1 e) drying (in the case of wet coating) PA1 f) firing.
In one particular enamelling technique, the enamel is applied to thin steel strip (0.3-0.4 mm) first in a double-sided base layer (drying/firing) and then in a single- or double-sided top layer (drying/firing). The enamelled "semifinished product" may then be correspondingly cut to size and bonded to form sandwich elements (support materials: gypsum boards, fiber boards, PU foams, etc.). However, the enamelled strip cannot be sharply bent or even deep-drawn on account of the glasslike nature of its surface. So far as the pretreatment and uniformity of coating of unshaped steel sheets are concerned, major advantages are obtained over the conventional technique of treating previously shaped sheets. However, workpieces can only be shaped to a limited extent, if at all, after coating with enamel in view of the brittleness of the enamel layer.
U.S. Pat. No. 3,154,426 describes a process for enamelling metal objects. An enamel layer is first applied to the pretreated metal and dried, after which an organic polymer is applied and subsequently fired. The disadvantage of this process is that, in the case of light-colored enamelling, the metal sheets have to be intensively pickled and nickel-plated to obtain good adhesion. Although this can be done continuously on the coil itself either chemically or electrolytically, it does require expensive equipment.
In order to avoid the complicated procedure involved in the pretreated process described in U.S. Pat. No. 3,154,426, U.S. Pat. No. 3,906,125 proposes pretreating the steel sheets by various annealing processes. However, this procedure is also very complicated and expensive and, accordingly, has never been adopted in practice.